In oilfield exploration and production operations, various logging techniques are employed to characterize and explore downhole formations. For example, in monopole, dipole or other multi-pole sonic logging, the measurement is usually taken with an array of sensors distributed circumferentially and axially along a tool body. These sensors each receive a signal sent from a transmitter. The transmitter can be located on a different section of the tool or on the surface. The transmitter sends the signal through the formation to the sensors in the tool.
The radial distance between opposite sensors distributed uniformly circumferentially is used as a fundamental parameter to determine the sensitivity of the measurement. The radial distance is determined by the diameter of the circle that the sensors form at a given tool axial location. Non-axis symmetry modes, such as dipole and quadrupole modes, are particularly sensitive to this parameter. A dipole field is generated by two opposite polarity sources a fixed distance apart. In this context, the magnitude of the dipole field propagating along the borehole can be measured with pairs of sensors located on opposite sides of the tool. Similarly, a quadrupole field involves four polarity sources, and in this context the magnitude of the quadrupole field involves four sensors equally spaced along the circumference of the circle that the sensors form at a given axial location.
An important factor determining the quality of the measured fields is noise suppression. Noise is generally unwanted. For instance noise may be generated due to direct interaction between tool hardware and borehole wall, or other random occurrences in the borehole in which this noise may be perceived by the sensors when measuring. Road noise suppression by both hardware design and processing are key challenges of tool configuration. Such noise generated by tool hardware tapping the borehole wall propagates inside of borehole often been seen as coherent energy wave as if it is signal comes through formation. Noise is typically minimized by avoiding tool from direct contact from borehole while measurement is in progress. Noise can also be suppressed through filtering and data processing means. For example, a dipole source transmits at a specific frequency, depends on borehole size and formation velocity, in sonic logging typically within 0.3 to 5 kilohertz, and the sensor then records all frequencies. However, because expected frequency and energy propagation mode is known, other signals of other frequencies and propagation mode can be filtered out during processing.
Referring now to FIG. 1, a typical variation of a peak amplitude of a dipole pressure field 130 across a borehole is shown. In this figure, the sensor locations of the circumferentially distributed sensors 120 are shown centered about the tool axis 100 and positioned away from the borehole wall 110. As can be seen, the amplitude of the dipole pressure field 130 drops when moving from a borehole wall 110 to the center of the borehole.
Referring now to FIG. 2, another variation of a dipole peak amplitude of a dipole pressure field 230 across a borehole is shown. With the same radial positioning of the sensors 220 about the tool axis 200, as with FIG. 1, and a larger borehole diameter, the distance between the sensors 220 and the borehole wall 210 is increased. As stated previously, the dipole pressure field 230 decreases when moving from the borehole wall 210 to the center of the borehole 200. Larger borehole diameters usually exist at the top of the well. As such, the formation shear speed is typically much slower than the mud speed. These conditions lead to the amplitude of a dipole pressure field dropping more rapidly moving from the borehole wall 210 to the center of the borehole 200.
Current wireline tools usually have an outer diameter smaller than four inches (making the radial distance between sensors even less). This limited radial separation of the sensors makes sonic logging difficult in large boreholes, especially at the top of the well. As shown in FIGS. 1 and 2, the smaller tool diameter to borehole diameter ratio results in smaller signals at the sensor locations. Additionally, it may be difficult to keep the tool centered in large diameter boreholes, in which tool eccentering may result in mode contamination of the received signal. For example, if the tool is centered in the borehole, a dipole logging signal may be obtained by subtracting the signals from two diametrically opposite sensors at a tool axial location. If the tool is eccentered, the subtraction will result in a certain amount of monopole signal. These “leaked” monopoles, typically known as Stoneley signals, hinder an accurate extraction of the formation shear speed from the recorded signal. Further, if more than two circumferential sensors are used to obtain the dipole pressure field, the mode “leakage” can be exacerbated.
A transmitter source is also typically constrained to fit within the outer diameter of the tool. As a result, a large portion of the acoustic energy excited by the transmitter may be lost in the mud surrounding the tool and within the casing, and thereby not exciting the borehole modes of interest. Like the sensor assembly, centering the transmitter source along the borehole axis may be difficult. Firing an eccentered transmitter generates unwanted modes. For example an eccentered dipole transmitter will excite a dipole field but will also excite monopole and other multi-pole fields in the borehole. Those unwanted modes make it harder to extract the originally wanted signal which is to be used to extract information on the rock. As such, there exists a need for a downhole tool to improve upon current tool eccentering and data accuracy.